1. Field of the Invention
This invention relates to a control system for an electrically heated catalyst of an internal combustion engine, which controls the operation of an electrically heated catalyst (hereinafter referred to as "the EHC") for purifying exhaust gases emitted from the engine, and more particularly to an electrically heated catalyst control system of this kind which monitors temperature rise, power consumption, or electrical resistance of the EHC, based on the voltage and current of power supplied to the EHC to thereby control the supply of power to the same, and detect abnormality of the same.
2. Prior Art
Conventionally, an electrically heated catalyst control system has been proposed by Japanese Laid-Open Patent Publication (Kokai) No. 6-81636, which comprises a catalyst temperature sensor formed e.g. by a thermocouple or a thermistor, for detecting the catalyst temperature of the EHC, detector means for detecting the operative state of the catalyst temperature sensor, switch means for making a changeover between the supply of power to the EHC and the cutoff of the same, and control means for controlling the operation of the switch means based on a signal delivered from the detector means. According to the proposed system, if abnormality of the catalyst temperature sensor, such as a disconnection and a short-circuit, is detected by the detector means, the control means operates to control the operation of the switch means such that the supply of power to the EHC is limited within a predetermined time period to prevent overheating of the EHC and complete heating failure of the EHC.
Further, an electrically heated catalyst control system is also known, which uses a voltage/current-sensing circuit for detecting the voltage and current of power supplied to the EHC to thereby monitor the operative state thereof.
FIG. 1 shows a typical voltage/current-sensing circuit employed in the known electrically heated catalyst control system.
In the figure, electric current I.sub.EHC flowing through the EHC is detected from a voltage drop V.sub.S across a shunt resistance R.sub.S serially connected between the EHC and a battery E.sub.B.
The former conventional electrically heated catalyst control system using the catalyst temperature sensor, such as a thermocouple, is required to be arranged in the catalyst in a high-temperature environment, which causes problems of degraded operating characteristics and/or a shortened life of the catalyst temperature sensor per se. Further, the catalyst temperature sensor is liable to become faulty due to a disconnection or a short-circuit in the wiring.
Although the latter control system using the voltage/current-sensing circuit is advantageous in that it need not detect the voltage V.sub.EHC and the current I.sub.EHC in the high-temperature environment, it suffers a problem that the voltage V.sub.EHC applied to the EHC decreases due to the voltage drop (V.sub.S) caused by the shunt resistance R.sub.S interposed between the battery and the EHC.
Further, since the electric current I.sub.EHC is set to a very large value, heat is generated by the shunt resistance R.sub.S, which causes a variation in the resistance value thereof, resulting in degraded detection accuracy of the voltage/current-sensing circuit. To overcome this inconvenience, it is necessary to employ a resistance which withstands a large power and has an accurate resistance value, leading to an increased manufacturing cost.